Treatment of pyrite and arsenophrite containing material with ferric ions and sulfur dioxide/oxygen mixture to improve extraction of valuable metals therefrom

ABSTRACT

A method of treating a mined material which includes a sulphide mineral and iron or a concentrate of the mined material to improve the recovery of a valuable metal from the sulphide mineral is disclosed. The method comprises: (i) oxidising the sulphide mineral in the presence of ferric ions to make the valuable metal in the sulphide mineral more accessible to extraction; and (ii) oxidising ferrous ions generated in step (i) or derived from iron in the mined material with a mixture of sulphur dioxide and oxygen to produce ferric ions for step (i).

The present invention relates to a method to enhance the extraction ofvaluable metals, such as gold and silver, from sulphide minerals.

The present invention relates particularly, although by no meansexclusively, to a method to enhance the extraction of valuable metalsfrom sulphide minerals that include iron or which form part of orebodies that include iron.

The present invention relates more particularly, although by no meansexclusively, to a method to enhance the extraction of gold from ironsulphide minerals, such as pyrite (FeS₂) and arsenopyrite (FeAsS).

A large part of the accessible gold mineral reserves and resources areassociated with sulphide minerals, such as pyrite and arsenopyrite. Inmany cases, these sulphides yield gold recoveries below 80% usingconventional cyanidation methods and, as a consequence, the sulphideminerals are classified as refractory. The main reason for the low goldrecoveries is that it is common to find some of the gold as very finelydisseminated particles inside the sulphide crystal matrix and thereforethe gold is not readily accessible by conventional cyanidation methods.The particle size of the gold inside the sulphide matrix can range froma few microns to submicrons.

In order to efficiently recover gold (as well as other metals) fromrefractory sulphide minerals it is first necessary to break down thesulphide minerals, for example by oxidising the sulphide minerals. Thegold can then be recovered by conventional gold recovery methods, suchas cyanidation treatment.

It is known to oxidise sulphide minerals by roasting and by a range ofhydrometallurgical methods including pressure leaching and bioleaching.

It is an object of the present invention to provide an alternativemethod of treating refractory gold-containing sulphide minerals based onhydrometallurgical oxidation of sulphide minerals to improve therecovery of valuable metals from the sulphide minerals.

According to the present invention there is provided a method oftreating a mined material or a concentrate of the mined material toimprove the recovery of a valuable metal from the mined material, themined material including a sulphide mineral which contains the valuablemetal, and the mined material including iron, which method comprises:

(i) oxidising the sulphide mineral in the presence of ferric ions toproduce ferrous ions, the purpose being to make the valuable metal inthe sulphide mineral more accessible to extraction; and

(ii) oxidising the ferrous ions generated in step (i) or present in themined material with a mixture of sulphur dioxide and oxygen to produceferric ions for use in oxidising the sulphide mineral in step (i).

The present invention is based on the realisation that:

(a) oxidation of a refractory sulphide mineral in the presence of ferricions to break down the sulphide mineral and produce ferrous ions;

(b) oxidation of ferrous ions by sulphur dioxide/oxygen to generateferric ions; and

(c) the use of the sulphide mineral or an ore body that contains thesulphide mineral as a source of the ferrous/ferric ions;

is an effective, industrially realistic, process for treating thesulphide mineral to improve subsequent recovery of a valuable metal fromthe sulphide mineral.

The valuable metal may be any metal, such as gold, nickel, copper andzinc.

It is noted that oxidation step (i) may make the valuable metal moreaccessible to extraction from the sulphide mineral by releasing solidparticles of the valuable metal from the sulphide mineral structure orexposing the valuable metal in the structure for dissolution of thevaluable metal. In either case, the valuable metal (in solid ordissolved form) can then be extracted by any suitable means. Forexample, in the case of gold in a sulphide mineral, oxidation step (i)typically would release the entrapped gold particles for extraction byconventional treatments, such as cyanidation. In addition, in the caseof nickel in a sulphide mineral, oxidation step (i) typically wouldbreak-down the sulphide mineral and expose nickel particles which wouldthen pass into solution and be extracted thereafter from the solution.

It is preferred that the concentration of ferric ions be maintainedabove a minimum level.

It is preferred that the ratio of the concentrations of ferric toferrous ions be maintained above a minimum level.

It is preferred that the method comprises oxidising the sulphide mineralin step (i) in the presence of a catalyst.

The catalyst may be any suitable material. A preferred catalyst issilver ions. The catalyst may be added to the mined material or theconcentrate. Alternatively, or in addition, the catalyst may be derivedfrom the mined mineral.

It is preferred that the iron be part of the sulphide mineral.

It is preferred particularly that the sulphide mineral be pyrite and/orarsenopyrite.

The term "pyrite" is understood herein to mean a non-living constituentof the earth's crust containing the compound iron sulphide having thechemical formula FeS₂.

The term "arsenopyrite" is understood herein to mean a mineralcontaining iron arsenic sulphide (FeAsS).

It is preferred that the oxidation of the ferrous ions be carried out ata temperature of at least 60° C.

It is preferred particularly that the oxidation temperature be at least80° C.

It is preferred that the oxidation of the ferrous ions be carried out ata ratio of sulphur dioxide to oxygen of 0.5-10%.

It is preferred particularly that the ratio of sulphur dioxide to oxygenbe 0.1-5%.

It is preferred more particularly that the ratio of sulphur dioxide tooxygen be 2%.

It is preferred that the oxidation of the ferrous ions and the sulphidemineral be carried out under acidic conditions.

It is preferred particularly that the oxidation of the ferrous ions andthe sulphide mineral be carried out at a pH of less than 3.

The concentrate of the mined material may be formed by any suitablemethod steps. Typically, the concentrate is formed by a combination ofcrushing/grinding and flotation steps.

The oxidation steps (i) and (ii) may be carried out simultaneously inthe same vessel or separately in different vessels.

The oxidation steps (i) and (ii) may be carried out on a continuous or abatch basis.

The oxidation step (i) may be carried out by percolation or by any othersuitable means of contacting a solution containing ferric ions and thesulphide mineral.

The sulphur dioxide used in the oxidation step (ii) may be provided fromany suitable source. For example, the sulphur dioxide may be provided byburning sulphur containing solids to produce sulphur dioxide. Otherpossible sources of sulphur dioxide are flue or stack emissions andliquid sources.

The oxygen used in the oxidation step (ii) may be provided from othersuitable sources, such as air.

According to the present invention there is also provided a method ofextracting a valuable metal from a sulphide mineral comprising:

(a) the treatment step or steps described in the preceding paragraphs;and

(b) an extraction step comprising extracting the valuable metal.

The valuable metal may be separated and recovered by any suitableextraction method steps.

Typically, where the valuable metal is gold, the gold is recovered bycyanidation.

Another option for the extraction of gold is by the use of a lixivantsuch as thiourea.

The method of the present invention is particularly, although by nomeans exclusively, adapted to enhance the extraction of gold frompyrite, arsenopyrite, and other gold bearing minerals that contain iron.

Without wishing to be bound by theory, the applicant believes that theoxidation of pyrite and ferrous ions follow the following stoichiometricreactions:

    FeS.sub.2 +8H.sub.2 O+14Fe.sup.3+ =15 Fe.sup.2+ +2SO.sub.4.sup.2- +16H.sup.+ Equation 1

    2Fe.sup.2+ +2SO.sub.4.sub.2- +SO.sub.2 +O.sub.2 =2Fe.sup.3+ 3SO.sub.4.sup.2- Equation 2

The applicant also believes that similar reactions apply to theoxidation of arsenopyrite.

It is noted from equation 1 that oxidation of pyrite by ferric ionsunder acidic conditions:

(i) breaks down pyrite so that valuable metals, such as gold, in thepyrite are in a more accessible form; and

(ii) produces ferrous ions.

It is noted from equation 2 that oxidation of ferrous ions by a mixtureSO₂ /O₂ produces a source of ferric ions to react with further pyrite.

In accordance with the present invention the pyrite itself provides asource of ferrous/ferric ions for equations 1 and 2.

A preferred flowsheet for the method of the present invention isillustrated in FIG. 1 and described in the following paragraphs in thecontext of pyrite.

With reference to the figure, a mined material which includesgold-bearing pyrite or a concentrate of the mined material is typicallyslurried at a predetermined pulp density with water and is supplied vialine 5 to a reaction vessel 3. The slurry is mixed in the vessel 3 bymeans of an agitator 9 and the temperature controlled via cooling orheating water supplied to the vessel 3 via line 7. Simultaneously, amixture of SO₂ and O₂ is sparged into the vessel 3 via line 11.

It is noted that the slurry may be mixed in the vessel 3 by any suitablemeans other than the agitator 9, such as an air sparging draft tube. Inaddition, it is noted that the slurry temperature in the vessel 3 can becontrolled by any other method, such as jacketed tanks. Generally, it islikely that the slurry would need to be cooled in view of the fact thatthe reactions in the vessel 3 are exothermic and would be expected togenerate more heat than can be utilised.

In accordance with equations 1 and 2, the SO₂ /O₂ gas oxidises ferrousions in the slurry, which produces ferric ions, and the ferric ions andoxygen oxidise pyrite, which produces ferrous ions.

The oxidation of the pyrite breaks down the pyrite and makes goldparticles entrapped by the pyrite more accessible for subsequentrecovery

After a predetermined residence time, slurry from vessel 3 istransferred successively via lines 13 to reaction vessels 15, 17 and istreated further in these vessels to break down any pyrite retained inthe slurry. Specifically, SO₂ /O₂ is sparged via lines 11 into theslurry in each vessel 15, 17 and agitators 9 mix the slurry in eachvessel 15, 17.

It is noted that any suitable number of successive reaction vessels maybe used to break down pyrite in the slurry.

By appropriate selection of a range of parameters in each vessel 3, 15,17, such as pulp density, retention time temperature, SO₂ /O₂ flowrates, and pH, for any given mined material it is possible to operatethe method utilising iron in the pyrite as the source of ferrous/ferricions and to continuously regenerate the ferric ions by SO₂ /O₂oxidation.

The slurry from the vessel 17 is transferred via line 21 to asolids/liquid separation tank 23. The solids stream from tank 23, whichincludes gold particles, is transferred via line 25 to a gold recoverytreatment, such as cyanidation, and the liquid stream from tank 23,which includes ferrous/ferric ions, is transferred via line 27 forsubsequent treatment or, optionally, for recycling to the vessels 3, 15,17.

The applicant carried out a series of experiments on pyrite toinvestigate the effect of experimental conditions, such as SO₂ partialpressure, pulp density, and temperature, on oxidation of pyrite byferric ions.

The experimental set-up consisted of a 1 liter five neck reactorimmersed in a water bath under temperature control. A stirrer wasprovided to mix the contents of the reactor. The stirrer was connectedto a variable speed motor.

Pyrite crystals were ground down to -325 mesh (45 μm), and solutionswere prepared using reagent grade chemicals. The pyrite was over 98%pure pyrite from Huanzala, Peru and was received as large crystals 7.5cm in size.

The following is a description of the steps taken for each experiment.

(i) 700 ml of a solution of 0.1M ferrous sulfate 1.0M sulfuric acid wasprepared using distilled-deionized water, and standardized by potassiumdichromate titration. The ferrous sulphate was used as a convenientsource of ferric ions.

(ii) The reactor was filled with the solution, and after it reached thedesired temperature a gas mixture was sparged into the reactor.Depending on the experiment, the gas mixture comprised O₂ or SO₂ /O₂

(iii) The gas mixture sparging continued for 1 to 2 hours prior to theaddition of pyrite in order to saturate the solution (conditioningperiod).

(iv) Just before pyrite was added, a solution sample was taken in orderto determine the ferric ion concentration and to confirm thestandardization of total iron.

(v) One neck of the reactor was opened and the ground pyrite mineral wascarefully put into the solution. Immediately the stirring speed wasincreased to 500 rpm to ensure full suspension of particles.

(vi) Samples were taken at predetermined periods of times. The samplingfrequency was higher for the first 2 hours of each experimental run.

The results of the experiments are illustrated in FIGS. 2 to 4.

The effect of the gas composition on pyrite oxidation was investigatedin experiments carried out at 80° C. with concentrations of 0%, 2%, and4% SO₂. The results are shown in FIG. 2.

The figure shows that the rates of pyrite oxidation were constant withrespect to time for each experiment. However, a significant increase ofthe oxidation rate was caused by the presence of SO₂ in the gas mixtureand the maximum rate was obtained at 2% sulfur dioxide.

The gas composition of 2% SO₂ produced the optimum conditions for theoxidation of ferrous sulfate in acid solutions at 80° C. After 60minutes of solution conditioning, over 90% of its iron was oxidized toferric ions. As pyrite was added, ferric ions were consumed by pyriteoxidation and regenerated by ferrous ion oxidation with the SO₂ /O₂ gasmixture. At these experimental conditions, the regeneration rate offerric ions was always higher than its consumption rate, and thereforethe ferric ion concentration was always high, above 98% of total iron insolution.

After 90 minutes of solution conditioning, with a gas composition of 4%SO₂, iron in solution was oxidized to ferric ions. As pyrite was added,ferric ions were consumed by pyrite oxidation at a faster rate thanregeneration by the gas mixture. As a consequence, the ferrous ionconcentration increased throughout the experiment while the oxidationrate decreased.

The effect of temperature on the pyrite oxidation rate was studied at60° C. and 80° C. using a 2% SO₂ gas mixture. The results obtained arepresented in FIG. 3. The figure shows that at temperatures of 60° C. and80° C. constant oxidation rates were observed throughout theexperiments.

The changes of iron species concentrations over time were compared forboth experiments. At 60° C., after 1 hour of solution conditioning,about 15% of the total iron in solution was oxidized to its ferric form.When pyrite was added, the ferric ion concentration decreased veryquickly to a concentration below 0.01M (10% of total iron in solution).

At 80° C., after conditioning the solution for 1 hour, most of the ironin solution was present in the ferric form, and the ferric ionconcentration increased as pyrite was oxidized, keeping the ferrous ironconcentration always below 0.01M.

The effect of pulp density on pyrite oxidation rate was studied in therange from 5 to 20 grams FeS₂ /liter. The results obtained are shown inFIG. 4. The figure shows that the rates of oxidation of pyrite per unitmass were substantially equal and constant with respect to time for therange of pulp densities studied.

The applicant also carried out a series of experiments on ferroussulphate solutions as a convenient source of ferrous ions to investigatethe effect of experimental conditions on oxidation of ferrous ions bygas mixtures containing SO₂ /O₂.

The experimental set-up consisted of a laboratory size reactor with astirrer controlled by a variable speed motor. The experiments werecarried out on a batch basis under the following constant conditions:

(i) solution temperature=80-85° C.;

(ii) solution concentration=0.1M ferrous solution;

(iii) solution volume=1.5L; and

(iv) stirrer speed=750 rpm;

The oxidation rates were obtained by monitoring either the ferrous ionconcentration or the ferric ion concentration in solution over time andperforming regression analysis on the initial 60 minutes of thereaction. The slope of the linear regression was considered to be thereaction rate. A standard 0.01M potassium dichromate solution was usedto titrate for the ferrous ion and a standard 0.01M EDTA solution wasused to titrate for ferric ion concentration.

The results of the experiments are illustrated in FIGS. 5 to 7.

FIG. 5 is a plot of oxidation rates as a function of the sulfuric acidconcentration. The curve indicates that an optimum acid concentrationexisted for the experiments. The optimum appeared to be at 0.25Msulfuric acid concentration. The exact position of this optimum was notclear from the tests but is certain to lie within the 0.1M to 0.3M rangefrom the results in FIG. 5. The results indicate that the rate ofoxidation decreased as the acid concentration increased above theoptimum value.

FIG. 6 is a plot of oxidation rates as a function of oxygen flow rates.The steeper line on the figure indicates the change in oxidation rate asa function of pure oxygen flow rate. As the flow rate increased theoxidation rate increased linearly. The line of lesser slope illustrateshow the partial pressure of oxygen affected the oxidation rate. Oxygencomprises about 20.9% by volume of air. Thus the partial pressure ofoxygen in air is about 0.209 atm. This line also illustrates that as thenet oxygen flow rate increased the oxidation rate increased in a linearfashion. However, its lesser slope indicates that as oxygen partialpressure decreases so does the oxidation rate.

FIG. 7 shows the effect of changing the SO₂ /O₂ gas ratio on oxidationrates. The slope of each line is the oxidation rate. At an SO₂ /O₂ gasratio of 1% the oxidation rate was significantly lower than at a ratioof 2%. This result indicates that a 2% ratio was much more effective foroxidation than 1%. The results are consistent with the previouslydescribed results of studies into the oxidizing characteristics of anSO₂ /O₂ gas mixture in an acid solution.

The applicant also carried out an experiment on an ore body thatcontained iron sulphides to investigate the recovery of nickel from thesulphides. The iron sulphides were in the form of pyrite and pyrrhotite(FeS) and the experimental work was based on deriving ferrous/ferricions solely from the ore body--with no external addition of iron.

The experimental set up consisted of a laboratory size reactor with astirrer controlled by a variable speed motor. The experiment was carriedout on a batch basis under the following constant conditions:

(i) initial Fe concentration (M/1): 0;

(ii) initial H₂ SO₄ concentration (M/1): 0.12;

(iii) temperature: 81.5° C.; and

(iv) ratio of SO₂ :O₂ : 2%

A ground sample of the ore body was mixed with water to form a slurry.The slurry and acid were placed in the reactor and SO₂ /O₂ was spargedinto the slurry for a period of 14 hours. Samples were takenperiodically and analysed.

Over the course of the experiment there was a progressive breakdown ofthe sulphide minerals with, by way of example, 93% of the nickel passinginto solution. The breakdown was due to ferric ion oxidation of thesulphide minerals.

An important result of the experiment was that the ferric ions werederived wholly from the ore body. This result is reflected by FIG. 8which is a plot of the concentration of iron in solution against time.The figure shows that the concentration of iron increased at asubstantially constant rate over the first 8 hours of the experiment andthen levelled off.

In summary the experimental work carried out by the applicantestablishes the feasibility of oxidising a sulphide mineral by ferricions, oxidising the ferrous ions produced by the ferric ion oxidation ofsulphide minerals by SO₂ /O₂, and relying on the sulphide mineral or anore body which contains the sulphide mineral as a source offerrous/ferric ions.

Many modifications may be made to the preferred embodiment of thepresent invention without departing from the spirit and scope of thepresent invention.

What is claimed is:
 1. A method of treating a mined material or aconcentrate of the mined material to improve the recovery of a valuablemetal from the mined material, the mined material including a sulphidemineral, the sulphide mineral containing the valuable metal and iron andbeing pyrite and/or arsenopyrite, which method produces a treatedproduct and comprises:(i) oxidising the sulphide mineral in the presenceof ferric ions to produce ferrous ions, the purpose being to make thevaluable metal in the sulphide mineral more accessible to extraction;and (ii) oxidising the ferrous ions generated in step (i) or present inthe mined material with a mixture of sulphur dioxide and oxygen toproduce ferric ions for use in oxidising the sulphide mineral in step(i).
 2. The method defined in claim 1 comprising oxidising the ferrousions in step (ii) at a temperature of at least 60° C.
 3. The methoddefined in claim 1 comprising controlling the ratio of sulphur dioxideto oxygen to be in the range of 0.5 to 10%.
 4. The method defined inclaim 1 comprising oxidising the sulphide mineral and the ferrous ionsin steps (i) and (ii), respectively, under acidic conditions.
 5. Themethod defined in claim 1 comprising carrying out steps (i) and (ii)simultaneously in the same vessel.
 6. The method defined in claim 1comprising carrying out steps (i) and (ii) separately in differentvessels.
 7. A method of extracting a valuable metal from a minedmaterial or a concentrate of the mined material, the mined materialincluding a sulphide mineral containing the valuable metal, the sulphidemineral being pyrite or arsenopyrite, the method comprising:(i) treatingthe mined material or a concentrate of the mined material in accordancewith claim 1 to form a treated product; and (ii) extracting the valuablemetal from the treated product.
 8. The method defined in claim 1comprising oxidising the sulphide mineral in step (i) in the presence ofa catalyst.
 9. The method defined in claim 2 wherein the temperature isat least 80° C.
 10. The method defined in claim 3 wherein the ratio ofsulphur dioxide to oxygen is 2%.
 11. The method defined in claim 4comprising oxidising the sulphide mineral and ferrous ions in steps (i)and (ii), respectively, at a pH of less than 3.